1. Field of the Invention
The present invention relates to a conductive polymer solid electrolytic capacitor excellent in withstand voltage characteristic and impedance characteristic.
2. Description of the Background Art
A conductive polymer solid electrolytic capacitor employing a conductive polymer as an electrolyte exhibits an excellent impedance characteristic, and increasingly expands markets. An electrolytic capacitor is generally constituted of an anode metal prepared from a valve metal such as aluminum, tantalum or niobium, a dielectric film formed by an oxide film provided on the surface of the anode metal and a cathode formed through an electrolytic layer provided on the dielectric film. The electrolyte of the electrolytic capacitor has two important functions, i.e., a function of protecting and repairing the extremely thin oxide film and another function of drawing the capacitance from the dielectric film provided on the anode as a de facto cathode. A conductive polymer solid electrolytic capacitor typically employs a conductive polymer such as solid polypyrrole or a polythiophene derivative as an electrolyte. Such a conductive polymer has by far higher electric conductivity (i.e., electronic conductivity) as compared with a liquid electrolyte employed for a general electrolytic capacitor. Therefore, the capacitor employing this conductive polymer as the electrolyte can reduce the internal impedance, and exhibits excellent characteristics particularly as a high-frequency circuit capacitor.
However, the conductive polymer essentially has no ionic conductivity. Therefore, the capacitor employing the conductive polymer as the electrolyte is inferior to a conventional capacitor employing an electrolytic solution in repairability (i.e., anodic oxidation) for the oxide film of the electrolytic capacitor. Consequently, a conductive polymer solid electrolytic capacitor exhibiting a high withstand voltage cannot be manufactured. More specifically, a conductive polymer solid electrolytic capacitor employing aluminum for an anode withstands a voltage of about 16 V in practical use when subjected to 40 V chemical conversion, for example, and a conductive polymer solid electrolytic capacitor employing tantalum withstands a voltage of about 12 V in practical use when subjected to 24 V chemical conversion, for example. In the 40 V chemical conversion, a DC voltage of 40 V is applied when an oxide film of a dielectric substance is formed on the surface of the valve metal, and a capacitor exhibiting a withstand voltage of 40 V must be ideally obtained in this case. While the withstand voltage in practical use can be increased in principle by increasing the chemical conversion voltage, the capacitance is reduced as the chemical conversion voltage is increased in this case. Further, the withstand voltage in practical use is not increased in proportion to the chemical conversion voltage.
Typical electrolytic capacitors include an aluminum electrolytic capacitor employing aluminum as an anode metal and a tantalum electrolytic capacitor employing tantalum as an anode metal. In general, a porous electrode obtained by sintering tantalum powder is employed for the tantalum electrolytic capacitor. On the other hand, aluminum electrolytic capacitors include a chip-type electrolytic capacitor and a wound-type electrolytic capacitor. In order to manufacture the chip-type electrolytic capacitor employing a conductive polymer as an electrolyte, the electrolyte consisting of the conductive polymer is formed on an anode foil made of aluminum by electrolytic polymerization or chemical polymerization, carbon paste and silver paste are thereafter applied, and these components are stacked and dried for preparing a capacitor element. The chip-type electrolytic capacitor manufactured in the aforementioned manner has a remarkably excellent frequency characteristic, while it is extremely difficult to manufacture the element, disadvantageously leading to a high rejection rate. On the other hand, the wound-type electrolytic capacitor employing a conductive polymer as an electrolyte comprises an anode foil of aluminum provided with a dielectric oxide film on the surface thereof, a cathode foil and a separator provided between the cathode foil and the anode foil. After these components are wound, the electrolyte is formed by impregnating and polymerizing a monomer forming the conductive polymer.
While the separator is indispensable in order to prevent the wound-type electrolytic capacitor from a short circuit, the impedance characteristic of the capacitor is deteriorated due to the separator. In other words, the wound-type electrolytic capacitor, advantageous for increasing the capacitance, is inferior to the chip-type electrolytic capacitor in high-frequency characteristic.
As hereinabove described, the conductive polymer solid electrolytic capacitor generally has one of the two typical structures. However, it is difficult to obtain a high withstand voltage while maintaining a high-frequency characteristic without reducing the capacitance by increasing the thickness of a dielectric layer in either structure. In order to solve this problem, the inventors have already developed an electrolyte consisting of an ionic liquid and a conductive polymer (refer to pamphlet of International Patent Laying-Open No. 2005/012599). The inventors have found that an ionic liquid has an excellent function of anodizing a valve metal and can repair defects of an oxide film of aluminum, for example, for implementing an electrolytic capacitor exhibiting a high withstand voltage. However, a general ionic liquid, exhibiting excellent ionic conductivity, has no electronic conductivity. If a large quantity of ionic liquid is added in order to implement a capacitor exhibiting a high withstand voltage, therefore, the impedance characteristic of the capacitor is disadvantageously deteriorated. If the quantity of the ionic liquid is small, on the other hand, the withstand voltage is not improved although an excellent impedance characteristic is obtained in this case. In other words, it is extremely important for a solid electrolytic capacitor comprising an electrolyte consisting of an ionic liquid and a conductive polymer to attain both of an excellent withstand voltage characteristic and an excellent impedance characteristic, which are in trade-off relation.
On the other hand, a method of forming an electrolyte by previously impregnating an electrode foil with an ionic liquid and thereafter polymerizing a monomer for forming a conductive polymer is also disclosed (Japanese Patent Laying-Open No. 2006-24708). However, this document, describing that the capacitance and a capacitive impregnation ratio can be improved, gives no description as to implementability of an excellent withstand voltage characteristic.